In order to accurately control the rotational operation of an electric rotating machine, rotor position information of the electric rotating machine and information about a current flowing in the electric rotating machine are needed. Here, conventionally, the rotor position information is obtained by additionally attaching a rotational position sensor to the electric rotating machine. However, the additional provision of the rotational position sensor has a significant disadvantage in view of cost reduction, space reduction, improvement of reliability. Therefore, change from rotational position sensor style to sensorless style has been required.
Examples of control methods for changing the electric rotating machine from rotational position sensor style to sensorless style include a method of estimating the position of the rotor of the electric rotating machine mainly from an inductive voltage of the electric rotating machine, and a method of estimating the position of the rotor of the electric rotating machine by using saliency.
The inductive voltage which is used in the former method has a characteristic that the magnitude thereof is proportional to the speed of the electric rotating machine. Therefore, at a zero speed or a low speed, the inductive voltage decreases and an S/N ratio is deteriorated. As a result, it becomes difficult to accurately estimate the position of the rotor of the electric rotating machine.
On the other hand, in the latter method which uses saliency, in order to detect the saliency (positional dependence of inductance) of the electric rotating machine, a high-frequency voltage having a frequency different from a drive frequency of the electric rotating machine is applied to the electric rotating machine, a high-frequency current flowing in the electric rotating machine along with the application of the high-frequency voltage is detected, and the position of the electric rotating machine is estimated by using the fact that the magnitude of the high-frequency current varies depending on the position of the electric rotating machine owing to the saliency of the electric rotating machine.
Thus, the method using saliency has an advantage that the position of the rotor of the electric rotating machine can be estimated irrespective of the speed of the electric rotating machine, though a position estimation signal for estimating the position of the rotor of the electric rotating machine needs to be inputted to the electric rotating machine. Therefore, especially at a zero speed or a low speed, a position sensorless control method using saliency is used.
Conventionally, for example, techniques disclosed in Non-Patent Literature 1 and Patent Literature 1 shown below have been proposed as methods of estimating the position of the rotor of the electric rotating machine by using saliency.
That is, a conventional position estimation method disclosed in Non-Patent Literature 1 estimates the position by applying a high-frequency voltage to a γ-axis which is a given control axis. That is, a current flowing in the electric rotating machine owing to application of a high-frequency voltage to the γ-axis, which current has the same frequency component as that of the high-frequency voltage, is converted into a current idm on a dm-axis which lags by 45 degrees from the γ-axis, and a current iqm on a qm-axis which leads by 45 degrees from the γ-axis. Then, magnitudes Idm and Iqm of the currents idm and iqm are calculated by using Fourier transform, and proportional-integral control (PI control) is performed such that the magnitudes Idm and Iqm become equal to each other, whereby the position of the electric rotating machine is indirectly estimated.
Meanwhile, in the conventional technique disclosed in Patent Literature 1, in order to reduce noise caused by a voltage or a current having a frequency higher than a fundamental frequency applied to the electric rotating machine for estimating the position of the electric rotating machine, the frequency of the voltage or the current applied to the electric rotating machine, which frequency is higher than the fundamental frequency, is set to be random.    NON-PATENT LITERATURE 1: K. Ide, “Saliency-based Sensorless Drive of Adequate Designed IPM Motor for Robot Vehicle Application”, PCC-Nagoya, 2007, pp. 1126-1133    PATENT LITERATURE 1: Japanese Laid-Open Patent Publication No. 2004-343833